1. Field of the Invention
This present invention relates generally to cable monitors and, more particularly, to cable monitoring devices for concurrently monitoring the operation of a plurality of test leads, including both signal and reference leads.
2. Description of the Prior Art
In the use of electrocardiogram equipment, a patient monitoring system is typically arranged to utilize a multiconductor cable which extends from the EKG unit to the vicinity of the patient. By means of a cable connector, the wires within the multiconductor cable are connected to the skin of the patient via individual test leads. The test leads include a plurality of signal leads and usually a single reference lead. The test leads are attached to the patient by means of adhesive pads with or without snap-on connections for the test leads. The pads themselves include electrodes which contact the patient's skin. With each heartbeat of the patient, the skin condition of the patient's body causes a very small interchange of electrons between the signal lead and reference lead electrodes. This produces a very tiny signal in the test leads. In conventional systems, this signal, generated at the surface of the patient's skin, is used as a biasing signal to influence the magnitude of a larger externally generated electrical signal which is shaped and propogated to drive a CRT or pen recorder display.
A number of problems can arise between detection of a heartbeat by the test leads before a resulting signal arrives at the EKG unit. For example, the adhesive electrode pads attached to the patient may not be placed properly to receive the relatively small voltages generated. Also, the test leads are typically made of relatively fine wire to enhance their flexibility and can be broken within the overlying insulation without detection. Furthermore, the plugs and jacks used to connect the test leads to the connector are quite small and also may not make proper contact.
While these problems may occur with the test leads and pads, normally the cable connector and the multiconductor cable leading to the EKG unit itself have a relatively high degree of reliability and problems do not usually occur with the cable and connector. But, when the display on the EKG unit indicates improper electrical connections, there is usually no way to determine which test lead assembly is at fault without undue manual manipulation and examination of the test leads. Thus, there has been a need in the application of EKG units for a method of detecting faults arising in the test lead assemblies.
Conventional cable integrity checking systems have been devised to warn of any break or disconnection of the test leads. Such conventional systems employ an externally generated pulse, of low level, so as not to harm the patient, which is directed to the adhesive pad on one of a pair of test leads. These externally generated signals are produced at a high frequency, as contrasted with the rate of normal heartbeat, so that when a heartbeat occurs and the electrical condition of the skin is altered accordingly, the artifically and externally generated signal is altered according to the electrical condition of the patient's skin. That is, the external signal is amplified by the signal generated at the patient's skin and returned on one of a pair of test leads.
All present systems devised for this purpose have certain fundamental deficiencies. In particular, they all require the external generation of electrical pulses which are perodically gated to the electrode pad, reshaped by the electrical condition of the skin, passed through threshold detectors, and used as gating signals to generate an output to the CRT of the EKG. As a consequence, there is no continuous monitoring of the condition of the patient since the externally generated pulses are periodically gated through on the test leads. Also, an electrical path exists which, though protected, nevertheless is suitable to conduct inordinately large voltage pulses to the patient's skin, when malfunctions occur, and impress inordinately large voltages on the test leads.